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I have three information about a low-pass FIR filter.

  • pass-band edge frequency
  • pass-band ripple
  • stop-band ripple

I want to get the stop-band edge frequency I've been googling for the last hour to find a formula to get, but without success . can any tell me how can I calculate it if have those 3 values ? thanks in advance

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The question is what you really want to do. Because the answer to your question is that there is no way to exactly compute the stopband edge from the three values you have. Looking at your specs I guess you're talking about a Parks-McClellan equiripple FIR filter, for which there are heuristic formulas relating the width of the transition band and the maximum ripple to the filter order (see this answer).

Usually in a design problem the passband and stopband edges would be given, and the maximally allowable error (ripple) would be given also, and you would need to figure out (by trial and error and/or by some heuristic formulas) what the required filter order would need to be. The problem you asked about doesn't occur in practice.

As a last comment, if you were given the actual filter coefficients then, by definition, the stopband edge is of course the lowest frequency for which the filter's magnitude response equals the given stopband ripple value (for a low pass filter, that is).

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